1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an in-plane switching liquid crystal display (IPS-LCD) device and a method of fabricating the same.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes two substrates, which are spaced apart and face each other, and a liquid crystal layer interposed between the two substrates. Electrodes are formed on the both substrates and face each other. A voltage is applied to each of the electrodes, and an electric field is induced between the electrodes. An arrangement of liquid crystal molecules is changed by varying the intensity of the electric field, and the transmittance of light is varied due to the arrangement of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image can be produced.
Of the different types of known liquid crystal display (LCD) devices, twisted nematic (TN) liquid crystal display devices are widely used. In a TN LCD device, liquid crystal molecules are parallel to two substrates and are arranged to be twisted between the two substrates, so that the liquid crystal molecules adjacent to the two substrates are perpendicular to each other. The liquid crystal molecules are driven by an electric field, which is induced between a pixel electrode of a lower substrate and a common electrode of an upper substrate. The electric filed is applied to be perpendicular to the upper and lower substrates. When a voltage is not applied, the liquid crystal molecules are arranged parallel to the substrates. On the other hand, when a voltage is applied, the liquid crystal molecules are arranged perpendicular to the substrates. However, the TN LCD devices provide a narrow viewing angle because of the longitudinal electric field. In order to solve such a problem, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. In the IPS-LCD device, both a pixel electrode and a common electrode are configured to be formed on the same substrate.
FIG. 1 is a cross-sectional view illustrating an IPS-LCD device according to the related art. As shown in FIG. 1, a lower substrate 10 and an upper substrate 20 are spaced apart from each other. A thin film transistor T, which includes a gate electrode, a semiconductor layer, a source electrode and a drain electrode, is formed on an inner surface of the lower substrate 10. A common electrode 12 and a pixel electrode 14 are formed on the inner surface of the lower substrate 10 in a pixel region and are spaced apart from each other. Although not shown in the figure, the common electrode 12 and the pixel electrode 14 are parallel to each other, and the pixel electrode 14 is electrically connected to the drain electrode of the thin film transistor T. A passivation layer 16 covers the thin film transistor T, the common electrode 12 and the pixel electrode 14.
The common electrode 12 and the pixel electrode 14 are formed of different layers, and the common electrode 12 and the pixel electrode 14 may be formed of the same layer. In addition, the pixel electrode 14 is formed of the same layer as the source and drain electrodes of the thin film transistor T as shown in FIG. 1. The pixel electrode 14 may be formed on the passivation layer 16.
A black matrix 22 is formed on an inner surface of the upper substrate 20, and the black matrix 22 has an opening corresponding to the pixel region. A color filter layer 24 is formed on the inner surface of the upper substrate 20 and corresponds to the opening of the black matrix 22. The black matrix 22 also corresponds to the thin film transistor T to prevent photo leakage currents in the thin film transistor T and blocks the light in a region excluding the pixel region. The color filter layer 24 includes red, green and blue color filters, and each color filter corresponds to the pixel region. The color filter layer 24 partly covers the black matrix 22, and thus the borderline between adjacent color filters is disposed on the black matrix 22.
An overcoat layer 26 is formed on the color filter layer 24. The overcoat layer 26 protects the color filter layer 24 and prevents a material of the color filter layer 24 from gushing out. The overcoat layer 26 also flattens the surface of the upper substrate 20 including the color filter layer 24.
A liquid crystal layer 30 is interposed between the lower substrate 10 and the upper substrate 20. Liquid crystal molecules of the liquid crystal layer 30 are arranged parallel to the lower and upper substrates 10 and 30. Although not shown in the figure, alignment layers are formed on the lower and upper substrates 10 and 30 adjacent to the liquid crystal layer 30. The alignment layers are arranged by a rubbing method, for example, and determine an initial arrangement direction of the liquid crystal molecules.
Lower and upper polarizers 40 and 50 are disposed at outer surfaces of the lower and upper substrates 10 and 20, respectively. A light transmissive axis of the lower polarizer 40 is perpendicular to a light transmissive axis of the upper polarizer 50.
In the IPS-LCD device, when a voltage is applied to the common electrode 12 and the pixel electrode 14, an electric field parallel with the substrates 10 and 20 is induced between the electrodes 12 and 14. Then, the liquid crystal molecules of the liquid crystal layer 30 are arranged to be parallel with the electric field and perpendicular to the electrodes 12 and 14.
As described above, the pixel electrode and the common electrode in the IPS-LCD device are formed on the same substrate, and a horizontal electric field parallel with the substrate is induced between the electrodes. In turn, the liquid crystal molecules are arranged to be parallel to the horizontal electric field, to thereby increase viewing angles of the LCD device. However, there may be a light leakage in the IPS-LCD device when a black state is formed, and thus a contrast ratio is lowered.
FIG. 2 is a view illustrating simulations of contrast ratios of a related art IPS-LCD device in a black state. Here, a light transmissive axis of a lower polarizer is parallel to a length of a liquid crystal panel and corresponds to the x-axis in FIG. 2, and a light transmissive axis of an upper polarizer is parallel to a width of the liquid crystal panel and corresponds to the y-axis in FIG. 2. In addition, an optical axis of a liquid crystal layer is parallel to the light transmissive axis of the upper polarizer. As shown in FIG. 2, there are light leakages at angles of about 45 degrees, 135 degrees, 225 degrees and 315 degrees, which are diagonal directions of the liquid crystal panel, in the black state, and the brightness of the IPS-LCD device is increased. Accordingly, the contrast ratio of the IPS-LCD device is lowered. This problem occurs due to the polarizers.
FIG. 3 is a cross-sectional view schematically illustrating a related art polarizer. As shown in FIG. 3, the related art polarizer 70 includes first and second protective films 74a and 74b and a polarizing film 72 between the first and second protective films 74a and 74b. In general, the polarizing film 72 is formed of polyvinyl alcohol (PVA), and the first and second protective films 74a and 74b is formed of triacetyl cellulose (TAC).
The polarizer 70 of FIG. 3 may be attached to either upper or lower surface of a liquid crystal panel by using an adhesive layer. Here, the protective films adjacent to the liquid crystal panel may have phase retardation, and may include a −C plate (negative C plate), in which nx=ny>nz, wherein nx is a first refractive index according to x-direction, ny is a second refractive index according to y-direction, nz is a third refractive index according to z-direction.
A polarizer is classified into O-type and E-type, and the O-type polarizer is widely used because the O-type polarizer has more properties in the black state as compared with the E-type polarizer.
When two O-type polarizers are arranged such that light transmissive axes are perpendicular to each other, a light transmittance is determined by the following equation:
                    T        =                ⁢                              1            2                    ⁢                                                                                                        O                    1                                    ⟶                                ·                                                      O                    2                                    ⟶                                                                    2                                                            =                    ⁢                                    1              8                        ⁢                                                            sin                  2                                ⁢                2                ⁢                ϕ                ⁢                                                                  ⁢                                  sin                  4                                ⁢                θ                                                              (                                      1                    -                                                                  cos                        2                                            ⁢                      ϕ                      ⁢                                                                                          ⁢                                              sin                        2                                            ⁢                      θ                                                        )                                ⁢                                  (                                      1                    -                                                                  sin                        2                                            ⁢                      ϕ                      ⁢                                                                                          ⁢                                              sin                        2                                            ⁢                      θ                                                        )                                                                    ,            wherein {right arrow over (O1)} and {right arrow over (O2)} are vectors illustrating polarizing directions of the polarizers, and φ and θ are directions of a viewing angle. In FIG. 4, φ and θ are defined as an azimuthal angle and a polar angle in xyz polar coordinates, respectively.
FIG. 5 illustrates light transmittances of orthogonal polarizers resulting from the above equation according to an azimuthal angle φ and a polar angle θ. As shown in FIG. 5, there is a light leakage around the polar angle θ of about 70 degrees, and the light leakage may be maximized at the azimuthal angle φ of about 45 degrees.
FIG. 6 is a view illustrating simulations of contrast ratios of the orthogonal polarizers in a black state using the above results. As shown in FIG. 6, there are light leakages at angles of about 45 degrees, 135 degrees, 225 degrees and 315 degrees. This is because orthogonality of the polarizers is broken according to viewing angles even if the light transmissive axes of the polarizers are perpendicular to each other.
As stated above, the light leakage increases the brightness of the IPS-LCD device in the black state, and thus the contrast ratio is lowered.